Electrical oscillation generator



y 9, 1940- D. J. FEWINGS 2,207,509

ELECTRICAL OSCILLATION GENERATOR Filed March 16, 1938 INVENTOR DAV/0JO/{A/ FEW/N68 ATTORNEY Patented July 9, 1940 UNITED STATES PATENTOFFICE David John Fewings, Chelmsford, England, assignor to RadioCorporation of America, a corporation of Delaware Application March 16,

1938, Serial No. 196,093

In Great Britain March 16, 1937 8 Claims.

This invention relates to electrical oscillation generators and moreparticularly to generators adapted to generate substantially sawtoothwaves and suitable for use as time base circuits for controlling thescanning action in cathode ray tube television and like apparatus.

It is common present day practice in time base circuits for cathode raytube television apparatus to employ sawtooth wave oscillators providingsawtooth waves of relatively small amplitude and then to amplify thewaves thus produced by means of a so-called paraphrase amplifier theoutput from which is employed to control the scanning action. Thispractice has a number of disadvantages. Firstly, a well smoothed hightension supply of somewhere about 1,000 volts and 30 milliamperes isgenerally required for the paraphase amplifier; secondly, if ordinarycommercially available valves of reasonably practical ratings areemployed in the amplifier, they are necessarily somewhat overloaded andconsequently have a short useful life; thirdly, commercially availablevalves do not present grid voltage-anode current characteristics ofsumcient rectilinearity over the large range of grid volts involved andin consequence the amplifier introduced a certain amount of distortionin the sawtooth waves; and lastly great difficulties are experienced inamplifying without losing some of the higher harmonic frequenciesinvolved in a sawtooth wave form of 10,000 cycles per second orthereabouts as employed in modern high fidelity television systems.

The object of the present invention is to provide apparatus wherebythese disadvantages are avoided, the said invention seeking to provide asubstantially sawtooth wave oscillator capable of producingsubstantially straight sided sawtooth wave oscillations of sufiicientamplitude to be employed for scanning control purposes without furtheramplification. As will be seen later, an oscillator in accordance withthis invention and employed in conjunction with a television or likecathode ray tube, may be arranged to be operated from the same hightension voltage supply source as is already provided for the cathode raytube.

The present invention provides a substantially improved sawtooth waveoscillator of the kind wherein the capacity portion of a resistancecapacity charging circuit is associated with a gasfilled electricdischarge tube whch breaks down and thus discharges the condenser when,or'just before, a predetermined voltage has been built up across saidcondenser and the said invention is characterized in that the saiddischarge tube comprises a main pair of electrodes between which thecondenser discharging discharges ocour, and an auxiliary pair ofelectrodes which are closer together than and positioned between themain pair of electrodes and between which an auxiliary discharge issetup. The auxiliary discharge may either be set up intermittently thearrangement being such that the setting up of the auxiliary dischargeinitiates the main discharge, or it may be a continuous discharge.

The invention is illustrated in and further explained in connection withthe accompanying drawing. i

Fig. 1 illustrates one embodiment of the invention.

Fig. 2, illustrates a second embodiment of the invention utilizing ahigh tension source.

Fig. 3 illustrates a third embodiment of the invention utilizing amagnetic coil for control purposes.

Referring to Fig. 1 which shows diagrammatically one way of carrying outthe invention, there is employed a tube I filled with argon or similarinert gas at a suitable pressure and containing two pairs of electrode2, 3; 4, 5, one pair 2, 3- hereinafter called the main pairbeing spacedapart by a substantial distance, and the other pair 4, 5-hereinaftercalled the auxiliary pairbeing more closely spaced and being situatedbetween the main pair. To take a practical example the distance betweenthe main pair 23 of electrodes may be such as to give a breakdownvoltage of about 1,000 volts and that between the auxiliary pair 4, 5,may be such as to give a breakdown voltage of volts. Each of theelectrodes of the main pair is earthed through one of two similarcondensers 0, 1.. The main electrode 2 is connected to the positiveterminal of a source 8 of supply through a resistance 9 and the othermain electrode 3 is connected to the negative terminal of the saidsource through a resistance l0. These resistances Band I0 should be asnearly as possible identical and may be adjustable. The source 8 ofpotential may conveniently be the same source as that which supplies thecathode ray tube with which the oscillator is to be associated and mayconveniently be a 6,000 volt source. The positive terminal of the sourceis connected to the auxiliary electrode 4 which is nearer to thepositive main electrode 2, through a resistance H, the negative terminalbeing connected to the other auxiliary electrode 5 through a furtherresistance l2. These two resistances H, l2, also should be similar andmay.

be adjustable. The positive auxiliary electrode 4 is earthed through acondenser I3 and the negative auxiliary electrode is earthed through asecond, similar condenser I4 which is preferably in series, as shown,with an adjustable resistance I5. A connection I6 for the application ofsynchronizing pulses is made to the junction of the condenser I4 andresistance I5. Sawtooth waves for application to a pair of deflectorplates in the associated cathode ray tube are taken off at terminals 26,21 through condensers IT, IS, from the main pair of plates 2, 3. Inpractice, a potentiometer resistance I9 with an earthed centre point 20may be connected across the source of potential 8 and the variouselectrodes of the associated cathode ray tube (not shown)- the Wehneltcylinder, the control electrode and the first, second, and thirdanodes-may be energized from individually adjustable tappings (2|, 22,23, 24 and 25 respectively) upon this potentiometer. This method ofconnection is normally quite practical, but in those cases in which thecathode ray tube forms part of a television receiver whose circuit issuch that one pole of the high tension supply unit must be earthy thecircuit above described may be modified by employing a high tensionsource whose total voltage is twice that required for the cathode raytube andmaking the various cathode ray tube tappings to different pointsupon one half of the potentiometer resistance. This is representeddiagrammatically in the accompanying Figure 2.

The apparatus in general operates as follows: The condenser 6 isconnected to the positive side of the source of potential 8 through aresistance 9 connected serially therewith. In turn, the condenser 6 isconnected serially with condenser I and resistor ID, the latter beingconnected to the negative side of the potential supply 8. The commonterminal of condensers 6 and I are connected to a point of referencepotential or, in other words, grounded. The same is true of the midpointof the potential supply 8, this being accomplished by grounding thecenter tap of resistor I9 which is shunted across the potential supply8. The main electrode 2 is connected to the common terminal of resistor9 and condenser E, and the main electrode 3 is connected to the commonterminal I0 and the resistor l. Assuming a condition of no charge oncondensers 6 and I, the side of the condenser 6 which is connected tothe resistance 9 will begin to charge up through the resistor 9 with acharge which leaves the condenser charged to a potential which graduallyapproaches the potential of the positive side of the potential supply 8.On the other hand, the potential of the side of the condenser which isconnected to the resistor II] will gradually pull down to approach avalue of the negative side of the supply 8. Hence the two condenserswill charge. These two condensers being connected serially, and theserial connection shunted across the main electrodes 2 and 3 willdischarge across the space discharge path between the main electrodes 2and 3 if the gaseous medium in the tube becomes conducting. Thecondenser I3 simultaneously begins to charge through the resistor I I toapproach the value of the positive terminal of the potential supply 8.

1 Hence the plate 4 begins to assume a positive potential. On the otherhand, the plate 5 will grow more negative in potential due to thecharging of condenser I4 through resistor I2, the side of the condenserI 4 which is connected to the resistor I2 being also connected to theelectrode 5, and tending to gradually assume a negative value of thenegative terminal of the supply source. Hence auxiliary electrodes 4 and5 will tend to assume gradually a high negative value.

Now initiation of discharge across plates 4 and 5 may be accomplishedeither by the impressing of a synchronizing signal on one of the platesby way of the connection IE, or the plates 4 and 5 may gradually assumea positive and negative value relative to each other whereby thepotential gradient in the discharge path between the elements 4 and 5exceeds the ionization potential of the gaseous medium. However, oncedischarge has been initiated between elements 4 and 5, then the mediumhaving been ionized will easily conduct between elements 2 and 3 or, inother words, 4 and 5 act as a trigger to cause conduction between theelements 2 and 3 and discharge condensers 6 and I.

In the arrangements of Figures 1 and 2, the two time constant circuits(each consisting 01 a resistance and capacity 9, 6 or ID, I in series)associated with the main electrodes are practically of identifical timeconstant, and similarly the two time constant circuits I I, I3 and I2,I4, I5 associated with the auxiliary electrode are also of practicallyidentical time constant. The time base constant circuits are sodimensioned that the potential between the main electrodes 2, 3, and thepotential between the auxiliary electrodes l, 5, reach their respectivebreakdown voltages at almost the same time so that the discharge betweenthe auxiliary electrodes will take place just when the discharge betweenthe main electrodes is ready to occur. Thus the auxiliary dischargeinitiates or triggers the main discharge. This action obviates thenatural irregularity in the high voltage discharge between the mainelectrodes which would otherwise occur due to the wide spacing of theseelectrodes. Further, it will be seen that the auxiliary discharge may beinitiated by the application of properly timed, properly polarized,synchronizing signals. Such signals which will maintain both dischargesin synchronism may be of much smaller amplitude than would be necessaryif the said signals were employed to initiate the main dischargedirectly.

As will be seen, the voltage upon one of the associated pair ofdeflector plates (not shown, but connected at 25, 21) in the cathode raytube will rise (during the long side of a sawtooth wave) fromapproximately earth potential to some predetermined positive potential(say 500 volts) and the other deflecting plate will, during the sametime, go from approximately earth potential to approximately the samepotential negative. In order to maintain the scan central on the screenof the cathode ray tube, suitable bias voltages-for the figures justgiven 250 volts positive and negative-should be applied to the deflectorplates with respect to the cathode ray tube anode. These bias potentialsmay be obtained from the potentiometer resistance 20 as indicated by thebroken line leads 2!], 38 of Figure 1, these leads containingresistances 3I, 32. The resultant scanning action will be such as toavoid trapezium distortion,

The resistance I5 which is in series with the condenser I4 between theauxiliary electrode 5 and earth controls the time occupied by theauxiliary discharge, and the value of this resistance should be sochosen that the auxiliary discharge continues for a short time after themain discharge has been extinguished. The advantageous result of this isthat when the potential between iliary discharge is continuous.

the main electrodes falls below that between the auxiliary electrodes,ions, and electrons comprising the main discharge are attracted into theauxiliary discharge and accordingly the main discharge path rapidly Inthe modification shown in Fig. 3, the condenser l3 of Figures 1 and 2and the series combination of condenser l4 and resistance l5 of the samefigures are omitted and the auxiliary electrode is connected to thepositive or negative terminal (as the case may be) of the supply sourcethrough a similar adjustable resistance H or 12 and the Wholearrangement is such that the aux- In this arrangement, the auxiliaryelectrodes preferably consist as shown of two rings disposed on a commonaxis extending between the main electrodes or one auxiliary electrodemay be a ring and the other may be of point form. The time constantcircuits associated with the main electrodes are so chosen that thedischarge between the said main electrodes can readily be initiated byenergy derived from a synchronizing signal energy being applied by meansof a coil 28 (or coils) suitably disposed about the discharge tube andarranged either to deflect or to concentrate the discharge between theauxiliary electrodes along the axis which constitutes the shortest pathbetween the main electrodes. This arrangement has the advantage that bycontrolling the potential difference between the auxiliary electrode,the amplitude of the sawtooth wave obtained can be controlled since thepotential diiierence between the main electrode can never fall belowthat between the auxiliary electrodes. Instead of using synchronizingsignals direct in the manner just described, they can be employed totrigger a small power high frequency oscillator to produce short burstsof high frequency oscillations which are.-

applied to the coil 28 disposed about the discharge tube so as toinitiate the main discharge. This arrangement gives a further control ofsawtooth wave amplitude since it enables the main discharge to beinitiated before the natural breakdown voltage is reached.

It is obviously of advantage to useas high a value of high tensionsupply voltage as possible (consistent with other practicalrequirements) for the higher the supply voltage in relation to thebreakdown voltage the better the rectilinearity. Quite goodrectilinearity can be obtained with a ratio of high tension voltage tobreakdown voltage of about 5:1 and a ratio of 8:1 (which, of course,gives better rectilinearity) is readily achieved.

What is claimed is:

1. A sawtooth wave oscillator comprising a pair of main electrodes, apair of auxiliary electrodes adjacent said main electrodes, saidauxiliary electrodes and said main electrodes being surrounded by anionizable gaseous medium, a source of potential having positive andnegative terminals, at least one time constant circuit connectedserially with said source of potential, said main electrodes beingconnected substantially in parallel with the capacitive element of saidtime constant circuit, and means for initiating ionization between saidauxiliary electrodes to cause ionization and conduction of said mediumbetween said becomes non-conducting;

main electrodes whereby the condenser element in said time constantcircuit is at least partially discharged.

2. Apparatus in accordance with claim 1, wherein the discharge betweensaid auxiliary electrodes is initiated intermittently from a signalreceived from an external source.

3. A sawtooth wave oscillator comprising at having positive and negativeterminals, a point of reference potential, resistive means shuntedsubstantially in parallel with said source of potential and having a tapthereon connected to said point of reference potential, a first timeconstant circuitcomprising a resistance and a condenser connectedserially to the positive terminal of said source of potential and to thepoint of reference potential, means electrically connecting one of saidmain electrodes to the common terminal of the resistance and condenserforming said first time constant circuit, a second time constant circuitcomprising a resistance and a condenser serially connected with eachother to the negative terminal of the source of potential and to ,thepoint of reference potential, means electrically connecting the other ofsaid main electrodes to the common terminal of the resistance andcondenser forming said second time constant circuit, and means forinitiating a discharge between said auxiliary electrodes wherebyionization occurs between said main electrodes and the charge on thecondensers included in said first and second time constant circuits areat least partially discharged.

4. Apparatus in accordance with claim 3 wherein there is provided, inaddition, an additional time constant circuit shunted in parallel withsaid first time constant circuit and having the common terminal of theresistance and condenser forming said additional time constant circuitelectrically connected to one of said auxiliary electrodes, and a secondadditional time constant circuit shunted substantially in parallel withsaid second time constant circuit and having the common terminal of theresistance and condenser forming said second additional time constantcircuit electrically connected to the other of said auxiliaryelectrodes.

5. Apparatus in accordance with claim 3 wherein initiationof dischargebetween the auxiliary electrodes occurs intermittently in response tosignals from a source external to said apparatus which are impressed onat least one of said auxiliary electrodes.

6. Apparatus in accordance with claim 1 wherein said time constantcircuit is variable.

'7. Apparatus in accordance with claim 3 wherein said time constantcircuits are variable.

8. Apparatus in accordance with claim 3 wherein the means for initiatingdischarge between said auxiliary electrodes comprises an electromagneticmeans positioned adjacent said auxiliary electrodes, ionizationoccurring when said electromagnetic means is energized.

DAVID JOHN FEWINGS.

